The present invention relates to magnetic iron oxide particles for magnetic recording, particularly to magnetic iron oxide particles comprising spindle-like magnetite or maghemite particles which are most suitable as the magnetic iron oxide particles for use in rigid disk recording, floppy disk recording and digital recording, uniform in particle size and shape, not contaminated by dendrites and have the small aspect ratio (major:minor) of less than 4:1, particularly 2:1, and a process for producing the magnetic iron oxide particles.
Recently, with the progress of long-time recording, miniaturizing and weight-saving of the reproducing apparatus for magnetic recording, the demands for the magnetic recording reproducing apparatus and the magnetic recording media such as magnetic tape, magnetic disks, etc., with higher performance and higher recording density has been intensified.
The fact is recognized from the following description in "Development of the magnetic materials and the technique for highly dispersing of the magnetic particles" published by SOGO-GIJUTSU-CENTER (1982) page 134. "The technical improvement which has been pursued up to now since the appearance of magnetic disk apparatus concerns the high recording density, and the high recording density has been rapidly raised as high as more than two figures within about 20 years in spite of the use of magnetic iron oxide particles of maghemite (.gamma.-Fe.sub.2 O.sub.3), as the magnetic material. The high recording density depends on, in addition to the improvement of .gamma.-Fe.sub.2 O.sub.3 particles, making the coating medium thin, the precision of the surface of the thus coated thin medium, the adoption of the technique of orienting the magnetic field, the avoidance of the floating-up of the head and the improvement of the specific characters of the head."
In order to form the magnetic recording medium with high performance and higher recording density, it is necessary to improve the dispersibility in the vehicle, loading in a coating medium, residual magnetic flux density (Br) and the smoothness of the surface of the magnetic tape, and to prepare the coating medium as thin as possible.
These facts are recognized from the following in "Development of the magnetic materials and the technique for highly dispersing magnetic particles", in page 140, "In the high recording density, it is necessary to increase the residual magnetic flux density (Br) for securing a predetermined output. In order to increase the residual magnetic flux density (Br), the orientation of the magnetic powder into the direction of the magnetic field must be increased and the degree of loading of the magnetic particles must be high." in page 141 of the same publication, "In order to obtain a high recording density, it is the most important factor to make the coating medium thin" in page 143 of the same publication, "The amount of floating-up of the head in the case of the head floating-up type such as in the rigid disk device is the major factor of the high recording density, and such a high recording density is possible by decreasing the spacing between the head and the recording medium. However,--in the case of the surface roughness not small enough, the reduction of the output signal occurs and it causes the headcrush due to the chipping of the head. Accordingly,--it is necessary to carry out the finishing of the surface of the coating medium as smooth as possible."
The specific properties of the medium for magnetic recording are close relationship to the magnetic iron oxide particles used for producing the magnetic media, and accordingly, the improvement in the specific properties of the magnetic iron oxide particles has been strongly demanded.
The relationship between the specific properties of the magnetic recording media and properties of the magnetic iron oxide particles used therefor are described more in detail as follows.
First of all, the residual magnetic flux density (Br) of the magnetic recording media depends on the dispersibility of the magnetic iron oxide particles in the vehicle, and orientation and loading of the magnetic iron oxide particles in the coating medium.
In order to improve the dispersibility in the vehicle and the orientation and the loading in the coating medium of the magnetic iron oxide particles, it is demanded that the particles to be dispersed in the vehicle are uniform in size and shape thereof and the particles are not contaminated by the dendrites and as a result, that the particles have large apparent density.
In the next place, in order to improve the surface properties of the magnetic recording media it is demanded that the magnetic iron oxide particles are excellent in the dispersibility and the orientation of the particles and the particle size is small enough, and as such magnetic iron oxide particles it is required that the particles are uniform in size and shape thereof and not contaminated by dendrites and as a result, that the particles have large apparent density.
In order to have the magnetic media of high performance and suitable for obtaining the high recording density, it is necessary that the coercive force of the media is as high as possible and is distributed in narrow range, and for that purpose, the coercive force (Hc) of the magnetic particles to be dispersed in the vehicle should be as high as possible and distributed in a narrow range.
Furthermore, in order to make the magnetic recording media as thin as possible, as is clearly seen in the following description of the above-mentioned reference, page 141 "For making the magnetic recording media as thin as possible, it is necessary to make the size of the magnetic iron oxide particle as small as possible and to give the excellent orientability in the coating medium. Formation of a thin coating medium is related to make a magnetic paint which is excellent in applicability by using the magnetic iron oxide particles with small oil absorption."
Those magnetic iron oxide particles of good dispersibility and orientability are favorable for that purpose, and as those magnetic iron oxide particles, those uniform in particle size and shape, and not contaminated by the dendrites are demanded.
On the other hand, one of the methods for forming the magnetic recording media with high recording density in the reproducing apparatus for magnetic recording is to narrow the width of magnetic head gap. This fact is clearly seen in the following description of the above-mentioned reference, page 15, "The important index representing the performance of the medium for magnetic recording is . . . the recording density. The improvement of the recording density has been carried out by improving the magnetic head and the recording medium.
The improvements which have been carried out in this field are mainly directed to realizing the magnetic head, the narrow width of the gap and the narrow width of the truck--."
The recording principle of the recording medium and the magnetic head in the longitudinal recording method (the method for recording a signal in the longitudinal direction of the magnetic layer) is the same as the following description in the above-mentioned reference, page 18, "In the ring head (FIG. 2a), a circular arc-like magnetic field is formed in the vicinity of the gap of the magnet center by the signal current of a coil. Since the magnetic field has a strong longitudinal component in the center of the gap, the medium is magnetized mainly in the longitudinal (within the plane) direction."
Although, for aiming the high recording density there is a tendency of narrowing the width of the gap of the magnetic head more and more in recent years, in such a case of narrowing the width of the gap of the magnetic head, the magnetic field in the vicinity of the gap in the magnetic center becomes to include the strong perpendicular component together with the longitudinal component. Accordingly, in the surface layer of the magnetic recording medium, which is in contact with the magnetic head, the width of distribution of the magnetic flux perpendicular to the medium becomes remarkably larger.
Consequently, in order to have a high recording density, it is preferable to make the medium having a direction of easy magnetization perpendicular to the medium within the magnetic recording medium.
As the representative magnetic particles hitherto, acicular magnetite particles or acicular maghemite particles are used, and in such cases, since the direction of easy magnetization is the longitudinal direction of the acicular particles due to the form anisotropy thereof, it is preferable to increase the perpendicular component by orienting the acicular magnetic iron oxide particles perpendicular in the coating medium or orienting thereof in random tridimensionally.
This fact is clearly seen from the following descriptions in Japanese Patent Application Laying-Open No. 57-183626(1982), "In addition, according to the introduction of perpendicular magnetic recording in recent years, there is a proposal of effectively using the residual magnetization component in the direction perpendicular to the surface of the magnetic recording medium. By the perpendicular magnetic recording, the recording density defined above becomes higher, and "In magnetic layer of the coating medium, . . . use of the magnetization component perpendicular or oblique (not parallel) to the magnetic surface . . . ".
In order to orient the magnetic iron oxide particles at random tridimensionally in the coating medium, thereby increasing the perpendicular magnetization component, it is effective that in addition to use of the particles uniform in particle size and shape, and not contaminated by the dendrites, the size of the magnetic iron oxide particles is reduced and the aspect ratio (major:minor) of the particle is reduced as possible to less than 4:1, particularly to less than 2:1, in other words, the particles show the shape magnetic isotropy and the magneto crystalline isotropy.
The just-mentioned fact is clearly seen from the following description in the above-mentioned Japanese Patent Application Laying-Open No. 57-183626/1982, "The present invention is characterized by the use of the particles of a small size of less than 0.30 .mu.m and . . . of the ratio of longitudinal length to transverse length of over one and below 3 in place of the conventionally used acicular particles having the major axis of 0.4 to 2 .mu.m or 0.3 to 1 .mu.m and the ratio of longitudinal length to transverse length of 5 to 20, . . . thereby suppressing the tendency of the particles in orienting within the coating medium due to the reduction of the thickness of the coating medium in the case of coating and drying or due to the flow of the coating material in the direction of the flow, and if necessary, allowing the increase of the perpendicular component of the residual magnetization positively."
At present, as the magnetic particles for magnetic recording, mainly the acicular magnetite or maghemite particles are in use. The acicular magnetic particles are obtained by the steps of reacting an aqueous solution of a ferrous salt with an alkali to obtain an aqueous solution containing colloidal particles of ferrous hydroxide, oxidizing the thus obtained colloidal solution showing a pH of higher than 11 by air (generally called as the wet process) to obtain the acicular (.alpha.-FeOOH) particles and reducing the .alpha.-FeOOH particles in a reductive gas such as hydrogen at 300.degree. to 400.degree. C. The thus obtained acicular magnetite particles are oxidized in air at 200.degree. to 300.degree. C. to obtain the acicular maghemite particles.
As has been described above, the magnetic particles which are uniform in particle size and shape, are not contaminated by the dendrites and have a small aspect ratio (major axis:minor axis) are those demanded most keenly at present, and in order to obtain the magnetic particles provided with such specific properties, it is necessary that the .alpha.-FeOOH particles (hereinafter referred to as "goethite particles") used as the starting material are uniform in particle size and shape, are not contaminated by the dendrites and have a small aspect ratio (major axis:minor axis).
Hitherto, the most representative and known process for producing the goethite particles in an alkali region of higher than pH 11 comprises the steps of adding an aqueous alkali solution in an amount of more than equivalent to an aqueous solution of a ferrous salt to obtain a solution containing the ferrous hydroxide particles and oxidizing the thus obtained solution at a temperature of lower than 80.degree. C.
The thus obtained goethite particles--exhibit the acicular form of an aspect ratio(major axis:minor axis) of larger than 10:1 and are contaminated by the dendrites, and from the viewpoint of their particle size, are not uniform in particle size.
By the way, as well known, the coercive force of the magnetic particles depends on the shape anisotropy, crystal anisotropy, strain anisotropy or exchange anisotropy, or on the mutual interaction of them. At present, the acicular magnetite particles or acicular maghemite particles are provided with a relatively high coercive force of 350 to 450 Oe by utilizing their anisotropy derived from their shape.
Though, the known acicular magnetite or maghemite particles have a relatively high coercive force, the coercive force thereof is further increased by incorporating cobalt to these particles while utilizing the crystal anisotropy due to cobalt. Further, it has been known that the coercive force of the particles is more increased as the amount of incorporated cobalt is larger.
As has been stated, those magnetic particles which are uniform in particle size and shape, not contaminated by the dendrites and small in the aspect ratio (major axis:minor axis) thereof are those demanded most keenly at present. However, the magnetic particles obtained from the acicular goethite particles as the starting material, which have been prepared by the above-mentioned known method are the acicular particles of the aspect ratio (major axis:minor axis) of larger than 10:1, contaminated by the dendrites and not uniform in size and shape.
On the other hand, the present inventors have obtained the information concerning the goethite particles used as the starting material for producing the magnetic particles for magnetic recording, particularly for rigid disk recording, floppy disk recording and digital recording. Namely, it is necessary that those goethite particles are uniform in particle size and shape, not contaminated by the dendrites and the aspect ratio of the particles is as small as possible, for example less than 4:1, particularly less than 2:1.
Then, the present inventors have noticed a process for producing the goethite particles by reacting an aqueous solution of a ferrous salt and an aqueous solution of alkali carbonate to obtain an aqueous solution containing FeCO.sub.3 and oxidizing FeCO.sub.3 in the aqueous solution by blowing an oxygen containing gas into the thus obtained aqueous solution, (refer to Japanese Patent Application Laying-Open No. 50-80999(1975)).
In this process, the thus obtained goethite particles are uniform in particle size and shape, and not contaminated by the dendrites and show the spindle-like shape.
However, since the aspect ratio (major axis:minor axis) of the thus obtained goethite particles is about 7:1, and it is demanded to reduce the aspect ratio of the particle.
On the other hand, in the case where the aspect ratio (major axis:minor axis) of the magnetic iron oxide particles is small, the coercive force of the thus produced magnetic iron oxide particles is lower than about 300 Oe because the anisotropy derived from the shape thereof does not act.
In addition, it has been known that the coercive force of the magnetic iron oxide particles of a small aspect ratio (major axis:minor axis) can be increased by the incorporation of cobalt thereinto, and in such a case, the thus obtained particles show not only the shape magnetic isotropy but also the magneto crystalline isotropy, namely the magnetic iron oxide particles for magnetic recording which are most keenly demanded.
However, it has been known that the magnetic iron oxide particles containing Co are poor in magnetic stability to the ambient temperature (particularly, the stability of the coercive force thereof to the temperature, hereinafter referred to as the "thermo-stability"), and there is a tendency that in the case where the content of cobalt is larger, the thermostability is poorer.
Concerning the instability of the magnetic iron oxide particles containing Co to the ambient temperature, for instance, FIG. 3 of Japanese Patent Application Laying-Open No. 48-87397 (1973) shows the situation thereof wherein a tendency of the reduction of coercive force of the magnetic iron oxide particles containing Co with the raise of the temperature thereof is indicated. More in detail, according to FIG. 3, the coercive force of the maghemite (.gamma.-Fe.sub.2 O.sub.3) particles containing Co is reduced from about 678 Oe at 25.degree. C. to about 420 Oe at 120.degree. C., thus showing a reduction of as high as 258 Oe.
Also in the case of the magnetite(Fe.sub.3 O.sub.4) particles containing Co, the coercive force thereof is reduced from about 572 Oe at 25.degree. C. to about 341 Oe at 120.degree. C., thus showing the reduction of as high as 231 Oe.
In addition, it has been known that the coercive force of the magnetic iron oxide particles is increased by coating the magnetic iron oxide particles by cobalt.
However, there is a demerit that the magnetic iron oxide particles coated by cobalt has a broader distribution of the coercive force than the particles not containing Co. This phenomenon is considered to be due, in the case where the particles are seen microscopically, to the different amount of cobalt between the particles and the fluctuation of the composition of cobalt even on the surface of each particle.
Accordingly, as a result of studying the process for producing the above-mentioned spindle-like goethite particles, which are uniform in particle size and shape, not contaminated by dendrites and have the aspect ratio (major axis:minor axis) of as small as possible, for example, of less than 4:1, particularly less than 2:1, the present inventors have obtained the following finding.
In the case where an aqueous solution of a ferrous salt is reacted with an alkali carbonate to obtain an aqueous solution containing FeCO.sub.3 and the thus obtained FeCO.sub.3 in the aqueous solution is oxidized by blowing an oxygen-containing gas into the aqueous solution containing FeCO.sub.3, by adding a water-soluble silicate in an amount of 0.1 to 20 atomic % (calculated as Si) to Fe (of the ferrous salt) into the aqueous solution of the ferrous salt, into the aqueous solution of alkali carbonate or into the aqueous solution containing the FeCO.sub.3 before blowing the oxygen-containing gas thereinto for oxidation thereof, the aspect ratio (major axis:minor axis) of the spindle-like goethite particles can become smaller, for example, less than 4:1, particularly less than 2:1, as compared to that in the case where the silicate is not added.
In addition, as a result of further studying of the process for producing the magnetic iron oxide particles showing a high coercive force and an excellent thermo-stability, the present inventors have obtained the following completely novel finding.
Namely, in the case where both the silicate and cobalt are added in the production of the spindle-like goethite particles as the starting material, the spindle-like magnetic iron oxide particles containing Si and Co which are obtained by reducing the goethite particles or reducing and oxidizing the goethite particles show the shape magnetic isotropy and the magneto crystalline isotropy thereof, and have a high coercive force together with an excellent thermo-stability.
Furthermore, as a result of further studying the process for producing the magnetic iron oxide particles showing a narrow distribution of the coercive force in addition to the high coercive force, the present inventors have obtained the following novel finding.
Namely, in the case where the spindle-like goethite particles containing Si are used as the starting material, spindle-like goethite particles containing Si are reduced or reduced and oxidized, thereby obtaining the spindle-like magnetic iron oxide particles containing Si and the thus obtained spindle-like magnetic iron oxide particles containing Si are coated by cobalt, the thus obtained coated particles have a high coercive force and in the same time, show a narrow distribution of the coercive force.
The present invention has been attained on the basis of the above-mentioned findings.